The present invention relates generally to the field of code division multiple access (CDMA) and time division multiple access (TDMA) wireless communications systems, and more particularly to methods of performing handoff of voice, data, or other communications in CDMA and TDMA telecommunications systems.
In CDMA wireless communications systems, base stations communicate with mobile units over a common bandwidth using quasi-orthogonal spreading codes to define individual communications channels.
In a TDMA system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, such as a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA xe2x80x9cframesxe2x80x9d having a predetermined duration. The number of time slots in each frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user.
In CDMA systems, because different channels transmitting in the same direction share the same bandwidth, the communications signals on the various channels interfere with each other and contribute to an overall system interference level. System capacity, or the number of active mobile units that can be supported within a given system bandwidth, is limited by acceptable system interference level.
Further, in cellular telecommunications systems such as CDMA or TDMA, as a mobile unit travels from one cell to another, service between the mobile unit and a given base station (or base stations) must be transferred to one or more xe2x80x9cnewxe2x80x9d base stations. This transfer process is termed xe2x80x9chandoff.xe2x80x9d Handoff is normally triggered by some threshold mechanism related to received signal strength, call quality, system loading, or other system parameters.
Handoffs can be xe2x80x9chardxe2x80x9d or xe2x80x9csoftxe2x80x9d. A xe2x80x9chardxe2x80x9d handoff is characterized by substantially simultaneous termination of communications with the xe2x80x9coldxe2x80x9d base station and initiation of communications with the xe2x80x9cnewxe2x80x9d base station. The mobile unit is in communication with only one base station at a time.
In a conventional xe2x80x9csoftxe2x80x9d or xe2x80x9cmake-before-breakxe2x80x9d handoff, the mobile unit and one or more xe2x80x9cnewxe2x80x9d base stations establish communications with each other before communications with the xe2x80x9coldxe2x80x9d base station (or base stations) are interrupted. Soft handoffs provide the advantage of xe2x80x9cspace diversityxe2x80x9dxe2x80x94utilization of signals from geographically distributed locations during handoff. This xe2x80x9cdiversity combiningxe2x80x9d provides improved signal coverage and quality.
However, since the mobile unit receives signals from multiple base stations during a soft handoff, the handoff requires the allocation of resources from multiple base stations. In a typical xe2x80x9csymmetricalxe2x80x9d soft handoff, in which diversity combining is provided on both the uplink (mobile to base station) and downlink (base station to mobile) directions, the provision of multiple signals also generates additional interference for other links.
Thus, the advantages of soft handoff are attained at the expense of increased backhaul requirements (i.e., the terrestrial or xe2x80x9cradioxe2x80x9d links that connect the base station to the network infrastructure), and greater bandwidth (for the air interface) and processor requirements. With high speed or wideband data such as video, the xe2x80x9ccostxe2x80x9d of multiple links becomes much more significant than with voice.
By way of example, the IS-95 Standard promulgated by the Telecommunications Industry Association and the Electronic Industries Association (TIA/EIA/IS-95) sets forth parameters for operation of conventional CDMA cellular telecommunications systems. As a design objective, IS-95 specifies a 2-way soft handoff zone at 30% of the service area, and a 3-way soft handoff zone at 10% of the service area. This requires allocation of a significant portion of system resources (radio, processors and other channel elements) to handoff.
Moreover, in conventional implementations of soft handoff, the xe2x80x9canchorxe2x80x9d points have limited processing capabilities and information available. As a consequence, the algorithms and performance are sub-optimal.
Accordingly, there exists a need for methods and systems that optimize uplink backhaul bandwidth and processor usage during handoffs.
It is therefore an object of the present invention to provide methods and apparatus that optimize uplink backhaul bandwidth and processor usage during handoff.
These and other objects of the invention will become apparant to those skilled in the art from the following description.
The foregoing objects, among others, are attained by the invention, which in one aspect provides base station controllers (BSCs), base transceiver subsystems (BTSs), mobile switching centers (MSCs) and radio connection controllers (RCCs) that cooperate to provide improved handoff. The terms BTS, RCC and MSC are used herein to denominate modules that execute known functions. Other labels can be used to designate these modules; and it is intended to cover any combination that executes the functions of the invention described herein.
In one aspect of the invention, the BTS performs a monitoring function, and determines whether it has valid data that originated from a mobile unit. In another aspect of the invention, the RCC performs the monitoring function, and determines whether it has valid dataxe2x80x94if it does not, it requests the BTS to send more information.
In one aspect of the invention, in a xe2x80x9csoft handoffxe2x80x9d involving a number xe2x80x9cnxe2x80x9d of BTSs (where n is greater than or equal to 2), the RCC executes the following operations:
(1) The RCC checks whether the information frame from any of the BTSs is valid. If 1 or more information frames is valid, then a valid frame is forwarded to a transcoder or data server or to some other network device. For simplicity, any reference to a transcoder, data server or to some other network device(s), is intended to include reference to any of these devices. If none of the received frames is valid, then the process continues at step 2 below.
(2) The RCC decodes the R=1/n code constructed from data received from the n BTSs.
(3) The RCC checks whether the decoded information frame is valid using the checksum or CRC transmitted with the information frame. If the information frame is valid, then this frame is forwarded to the transcoder or a data server. Otherwise, the system sends an invalid frame, indicated as such, or a xe2x80x9ccontrolxe2x80x9d frame indicating an xe2x80x9cerroredxe2x80x9d invalid frame was received.
The invention thus provides methods and apparatus to improve the performance of soft handoff in wireless systems, and thereby improve the capacity and user perception of these systems. These methods can also be used to provide and improve the performance of macroscopic diversity.
The methods can also be applied to inter-BSC, inter-MSC and inter-system handover, each of which constitute xe2x80x9cproblem scenariosxe2x80x9d in conventional implementations.
The invention will next be described in connection with certain illustrated embodiments and practices. However, it will be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.